RELATIONSHIP OF THE GPS ANTENNA PHASE CENTER TO EXPOSURE STATION
The GPS receiver collects the carrier phase and
pseudo-range information pertaining to aircraft throughout the photo mission.
the trajectory of the The raw data is post
processed into trajectory information consisting of GPS
time-tagged geocentric positions for center. This position is correlated station through time and orientation
the GPS antenna phase to the camera exposure of the spatial offsets
between two origins, earth center and photo center. rarely coincide with the times at which the antenna
the antenna position at
the time camera using an
and GPS antenna phase center and a elements of exterior orientation.
priori estimates for the These a priori estimates
may be refined during aerotriangulation and more accurate
exposure stations aerotriangulation
after each iteration of the
individual component observations can be process to yield a variance-covariance exposure station.
propagated during this matrix for the camera
INTERPOLATION ACCURACY AS A FUNCTION OF GPS SAMPLING RATE
Since it is not presently practical to control the GPS collection by the exposure event, the antenna phase center position at the time of exposure must be computed by interpolating the aircraft trajectory after the appropriate timing biases have been removed from the observed exposure time.
The exposure station position accuracy is a function of the kinematic positioning accuracy (1 or 2 cm relative accuracy for most geodetic quality GPS receivers), sampling rate and
interpolation model. sampling rates in part
GPS receiver manufactures have increased to enhance the accuracy of interpolation.
The inclination would be to sample the GPS receiver at as high a frequency as possible to minimize the time difference between
This practice also increases
select a exposure
sampling rate and interpolation model station accuracy commensurate with the
while at the same time minimizing post processing Operational experience gained by NOAA has demonstrated
that a 1 hertz GPS sample is as 1:10,000 when using the
adequate for photo scales as large interpolation model discussed in
Higher sampling rates or more different
influenced by short period turbulence. following analysis of the interpolation
To this end, the process is presented.
The GPS signals are generally sampled on a nearly uniform time interval affected only by a very small (one usec/sec) (King
subsequent to being clock drift removed
post-processed generally have receiver and are time shifted so that all antenna
positions are equally spaced in time.